There is a growing demand for low frequency vibratory sources to replace the impulsive sound sources in current use. Impulsive sound sources used for seismic exploration and monitoring such as dynamite or air guns have been blamed for harming the environment, marine mammals, and other sea life. A vibratory source such as a marine vibrator that can impart controlled vibrations over a period of many seconds as compared to milliseconds of an impulsive source has many advantages. Impulsive sound sources create significant broad band noise (up to 1 kHz) with much of it outside the useable seismic frequency band. Although frequencies above about 100 Hz are typically not seismically useful, they are blamed for the preponderance of the harm to marine life. A controlled vibratory source that can be limited to approximately 100 Hz would eliminate this concern. In addition, a precisely controlled vibratory source that is highly repeatable can improve imaging results as compared to impulsive sources which are less repeatable.
Designing a practical low frequency marine acoustic source capable of producing useable low frequency vibrations from 1 Hz to 100 Hz is very challenging. First to be of practical size, a marine vibrator must have dimensions much smaller than the wavelength of the sound that it produces. In water for example, at 100 Hz the acoustic wavelength is about 50 feet, but an acceptable size for a marine vibrator is probably three feet or less. Achieving high power at low frequencies from a small transducer requires very large volume displacements, up to hundreds of liters of water, which is at the very limits or possibly even beyond the limits of traditional technologies. For example, one current solution uses a magnetostrictive material (Terfenol-D) with a very high displacement. But Terfenol-D which has the highest magnetostriction of any known alloy provides only about 1/16 inch of displacement from a 3-foot stack. Despite this limitation, several companies have developed innovative flextensional transducers designed to leverage these small displacements into larger displacements, but flextensional transducers are heavy, expensive, and highly resonant. Alternatively, to overcome the small displacements of piezoelectric/magnetostrictive materials, flextensional transducers using voice coil actuators as drivers have also been developed for the marine vibrator application, but they have limited output and also have problems with resonant peaks. A Gas-Filled Bubble Seismo-Acoustic device was also developed for this application but it too cannot generate the signals anywhere near the bandwidth and power needed. Whereas these traditional technologies used for creating underwater sound may be well-suited for high and mid frequency applications, they simply do not scale when attempting to produce the low frequencies needed for seismic applications (i.e., 100 Hz and lower) and therefore have not resulted in a practical solution.